Casing pipes or patching pipes under an oil, gas or water well are always subject to deformation, wearing and diameter reduction during their use as a result of corrosion and crust movement, so that the well cannot work. The casing pipes are conventionally reshaped with the following technologies: reshaping using pear-shaped equipment, milling, and explosive reshaping. The reshaping using pear-shaped equipment includes lowering the pear-shaped equipment with a caisson down to where the casing pipe is deformed, and then expanding the casing pipe by impaction. The shortcoming of this technique is that the casing pipe's inside diameter will be smaller than the standard diameter, because elastic deformation may cause the expanded casing pipe to resile. The milling technique involves grinding or milling the deformed casing pipe. It may cause damage to the casing pipe. The common disadvantage of the two technologies above lies in a great labor intensity resulting from lifting and lowering the caisson for several times. The explosive shaping technique involves lowering explosives down to where the casing is deformed and then igniting the explosives to blast the deformed casing. However, the casing may also be damaged because of the explosion.
The applicant of this invention has disclosed a variable diameter expansion head in Chinese Patent Application No. 200620039368.X. The head is housed in a conical-nose enclosure. The main body of the enclosure is equipped with radial cylindrical holes to embed small steel balls, its internal longitudinal side is equipped with large steel balls and pads, and its bottom is equipped with springs. When a forward thrust overcomes the spring pressure, the small steel ball in front of the large steel ball will be pushed outwards by the large steel ball to where the expanded casing is deformed. In this way, the casing's diameter can be restored.
Well restoration has been partly solved by using the variable diameter expansion head with lower cost and good expansion effect.
However, there is still some deficiency existing in use of the expansion head. It is not a problem of expansion effect but that the head is subject to easy breaking in the process of expansion. Probably because a great down pressure is imposed on the steel ball in expansion and the inside of the enclosure is composed of the steel ball and the pad so that stress is centralized at the inside of the expansion head, whose structure is not tight enough.